1. Field of the Invention
The present invention relates to a method and apparatus for cutting a substrate using a coolant, more particularly, to a method and apparatus for cutting a selected portion of a substrate using a coolant having a high heat capacity to rapidly cool a heated portion of the substrate.
2. Description of the Related Art
Generally, a glass substrate is widely used in industrial, commercial and residential fields. The glass substrate is mainly made of silicon and has a non-crystalline structure.
In case that a minute groove is unexpectedly formed at a portion of the glass substrate, the non-crystal structure of the glass substrate causes a fine crack which is generated by impact or force applied to the minute groove. The crack is subsequently extended in an unexpected direction. Finally, this results in separation of an undesired portion.
This problem frequently occurs when using a method of cutting a glass substrate using a diamond cutter, in which a minute groove is formed on a surface of the glass substrate and then excessive stress is applied to the groove to separate the glass substrate. Although this cutting method might be suitable for commercial and residential fields, it is not suitable for the industrial field requiring high precision.
Particularly, the conventional cutting method is not suitable for cutting a glass substrate to be used for a liquid crystal display (LCD) device. To fabricate an LCD, it is necessary to integrate a plurality of semiconductor devices on a glass substrate having a desired surface area to form an LCD panel.
Recently, in order to maximize productivity of LCD panels, there has been proposed a technique in which 6 to 8 sheets of LCD panels are formed on a large-scaled mother glass substrate at the same time and then cut and separated individually from the mother glass substrate using the diamond cutting method. In this process, cutting an LCD panel from the large-scaled glass is very important, since the process belongs to a last step of the processes of fabricating the LCD panel.
When an LCD panel is separated from the large-scaled mother glass substrate using the diamond cutting method, as described above, a rough groove is formed on a cut surface due to the cutting characteristics of this method. In case that the LCD panel is separated from the large-scaled mother glass substrate along the rough groove, there is a critical problem that, since the cut surface is roughly formed, an undesired additional crack is generated and the crack is then spread, so that an image displaying portion is cut.
To solve the above problem, recently, there has been proposed a cutting method using thermal stress, in which a glass substrate is rapidly heated and cooled to cut a selected portion of the glass substrate.
In the cutting method using the thermal stress, which is different from the diamond cutting method, the portion to be cut is rapidly heated using laser beam having a desired wavelength. Then, the rapidly heated portion is rapidly cooled using a coolant having a temperature sufficiently different from that of the heated large-scaled glass substrate to form a scribe line that is partially cut. At this time, the scribe line functions to guide the portion to be cut so that a cutting region is precisely separated. Then, the laser beam heats again the scribe line so that the mother glass substrate is fully divided into multiple pieces due to thermal expansion.
In the above cutting method, an edge of the cut surface of the LCD panel is smoothly formed when the large-scaled mother glass substrate is divided into the multiple pieces. Therefore, it is prevented that the undesired crack is generated so that an undesired portion is separated due to the spreading of the crack. When a glass substrate is cut using the laser beam and the cooling fluid, the cutting performance may be improved or degenerated according to an energy level of the laser beam or a characteristic of the coolant.
Gas such as nitrogen and argon, etc., in a state of an ultra-low temperature is mainly used as a conventional coolant. However, when the gas is used as the coolant, the following problems occur:
Firstly, since the ultra-low temperature gas has a very small heat capacity, a temperature of the gas is changed when the gas is discharged. As a result, the gas having a higher temperature than a desired temperature is provided to the glass substrate. Therefore, it is difficult to efficiently cut the rapidly heated glass substrate, thereby degenerating the cutting speed and cutting characteristics.
Secondly, since the gas has the small heat capacity, it is difficult to lower a temperature of the glass substrate within a short time. Therefore, there is a problem of degenerating the cutting speed and the cutting characteristics.
Thirdly, the rapidly heated glass substrate and the discharged gas make movements relative to each other on the same plane. If the relative motion between the glass substrate and the gas is increased, it is difficult that the discharged gas precisely reaches a designated portion of the glass substrate, thereby decreasing the cutting speed.
Fourthly, when the low temperature gas is discharged, a surface area of the substrate to which the discharged gas reached is considerably larger than the area that the gas initially takes at the time of the discharge. As a result, the gas is partially lost in the air, so that a temperature difference between the heated glass substrate and the gas is reduced, thereby degenerating the cutting speed and the cutting characteristics.